1. Field of the Invention
The present invention relates to a method of forming composite images with use of a photosensitive member consisting essentially of three layers, i.e. an electroconductive layer, a photoconductive layer and an insulation layer.
2. Description of the Prior Art
In recent years, copying methods have been proposed wherein a composite image is obtained by forming a copy image from an optical image of a usual original and additionally forming another image by writing with an OFT or laser. For example, Published Unexamined Japanese Patent Application No. SHO 57-8553 discloses such a method, which comprises the following steps. In the first step, a photosensitive member is charged to a surface potential VS by a corona charger as illustrated in FIG. 1a. Subsequently in the second step which is shown in FIG. 1b, the charged photosensitive member is exposed to an optical image of a usual positive original, whereby the potential of the nonimage area is attenuated to VL, with the potential remaining approximately at VS in the image area. When the nonimage area is then exposed to a negative image with use of an OFT or laser in the third step, the potential VL is attenuated to VLL in an area corresponding to the image area as seen in FIG. 1c, whereby a composite electrostatic latent image is formed. The composite latent image thus formed is developed next in the fourth step, in which the developing bias voltage is set approximately to the above level VL, whereby two kinds of toners, opposite to each other in polarity, are adhered to the component latent images respectively as shown in FIG. 1d.
To obtain fog-free sharp composite images by the above copying method, it is critical that the potentials VS, V1 and VLL be stable. Of these, the potential VS can be provided always with good stability and ease, for example, by a scorotron charger. Stable potential VLL is available when a sufficient amount of light is used for positive image exposure. However, the intermediate potential VL between VS and VLL is often unstable because the sensitivity of the photosensitive member varies from member to member and is dependent on temperature and also because of variations in the amount of positive image exposure, etc. Difficulties are therefore encountered in setting the developing bias voltage. Although the above-mentioned publication No. SHO 57-8553 discloses means for controlling the amount of positive image exposure by detecting the potential VL, the control means makes the copying machine complex.
Furthermore, the copying method described has the drawback that in order to obtain the individual component latent images substantially with the same contrast, the intermediate potential VL needs to be approximately at the middle level between VS and VLL, thus necessitating delicate adjustment of the amount of exposure. The copying method has another drawback that when the negative image overlaps the positive image, the positive image substantially disappears. The method has still another drawback that when the image portion of the negative image is projected on the positive image in overlapping relation, both images disappear or the potentials of the two images offset each other and reduce, forming only a positive or negative image of low density.